My wife & I both have iPads, so for us the MiFi seems like the better option for us. Rather than pay $30/month each for two 3G iPads, we just pay the $60/month for the MiFi. In my area AT&T sucks, so we get more dependable service. Plus, if I want I can also use my laptop at the same time she's using her iPad.
She doesn't have wifi service at work, so she can bring the card with her during the day while I'm at home, and I just use our home service. Then when I go to work at night I just bring the card with me. Admitted that the MiFi doesn't have the greatest of battery life, but it still works up to 250ft. away while plugged in, so no biggie. Plus the car charger for my Blackberry works with the MiFi so charging isn't ever a problem.
I guess your decision shield depend on your particular situation, but for mine the MiFi was a much better choice.